[sdiy] Signals leaking into the PSU?

[A.M. Barrio]

 This is the complete email I wanted to send. There's another thread where
the attached files and additional info is missing. My bad, pressed CTRL +
Enter by mistake whoops. Please ignore that one! pata at ieee.org and
mbryant at futurehorizons.com, thank you for your replies, I have taken them
into account :-)

------------------------------------

Hello,
I'm having an issue where the frequency of my VCO is being affected by the
operation of other modules. I'll give a broader explanation:

I'm a beginner on synth DIY, right now I have my home made case, a PSU, two
VCOs, an LFO, a clock module and an amplifier/speaker (built in the case,
not externally). Everything has been arranged by me, I haven't purchased
any module yet. The clock and the LFO are the latest modules I have built
(pretty much both at the same time), and when I tested them on their own,
they worked just fine. However I noticed that when they are powered, the
frequency of the VCO would stutter following the operation of the clock and
the LFO.

That would be: *without connecting the clock or the LFO anywhere*, just
having them powered up, whenever the clock is up or down, or the waves
generated by the LFO reset, the frequency of the VCO varies a tiny bit. It
gets more noticeable the higher the pitch of the VCO.

After some head scratching I've realised that the signals generated by any
of the modules I have (both VCOs, the clock and the LFO) are somehow
leaking into the +12 and -12 rails of my PSU. Measuring any of the rails on
my scope (on AC mode, so only the noise is shown), I can see that the waves
generated by the modules I mentioned above are there as well in the form of
noise (around 20mVpp each of them). That explains the little variations in
frequency of the VCO. However I have no idea why this is happening or how
to fix it. I have tried with different PSUs I have around:


   - 11V 0.750A SMPS into DCDC to get +12 and -12
   - Two 12V 1A SMPS together to get +12 and -12
   - Linear PSU with 7812 and 7912

I have the same problem with all of them. I'm positive there's an issue
somewhere with a lack of filtering, but I don't know where, or why.

I'm attaching a sample of the VCO output where the stutter can be heard and
the schematic of the LFO. The way the LFO is set up in terms of caps and
isolation can be extrapolated to every other circuit I have made, I always
place those 100nF caps between each rail and ground.


 LFO (square).jpg
<https://drive.google.com/file/d/1ts1WGcyl2y1Ypp5eP79Ptb5fHXawetZh/view?usp=drive_web>

 VCO stutter.mp3
<https://drive.google.com/file/d/1RrUNX_Of_RGLAwn2CyV7I5fxRrlHvfHL/view?usp=drive_web>


In case you have any idea what could be wrong, your reply will be greatly
appreciated. Thanks beforehand!

Kind regards,

A.M. Barrio.

------------------------------------

In response to pata and Mike Bryant:

   - Yes, the current PSU I'm using can provide 8W in total. I'm only
   powering one VCO and one LFO with some LEDs, power shouldn't be the issue
   in this case.
   - I always place 100nF caps between each rail and GND on every
   circuit/board. However I hadn't heard of placing caps on each IC. I suppose
   it should be between their power pins and GND? I will have a look at
   capacitance multipliers as well as I haven't heard about them before.
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